Signal processing method for FM-CW radar

ABSTRACT

Disclosed is a signal processing method for an FM-CW radar that can accurately detect the relative distance, relative velocity, etc. with respect to a target approaching or receding at a high relative velocity, wherein predicted values for peak frequencies currently detected in upsweep and downsweep sections are computed from the previously detected relative distance and relative velocity, and it is determined whether any of the predicted values exceeds a detection frequency range and, if there is a peak frequency that exceeds the detection frequency range, the frequency is folded and the folded frequency is taken as one of the predicted values, the method then proceeding to search the currently detected peak frequencies to determine whether there are upsweep and downsweep peak frequencies approximately equal to the predicted values and, if such upsweep and downsweep peak frequency are found, the peak frequency approximately equal to the folded predicted value is folded and the folded peak frequency is used.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/584,178, filed on Jun. 23, 2006, now U.S. Pat. No. 7,391,361 which isa National Phase Patent Application of International Application NumberPCT/JP2004/019698, filed on Dec. 22, 2004, which claims priority ofJapanese Patent Application Number 2003-435084, filed on Dec. 26, 2003.

TECHNICAL FIELD

The present invention relates to a signal processing method for an FM-CWradar and, more particularly, to a signal processing method for an FM-CWradar which can accomplish correct pairing even when an upsweep or adownsweep peak frequency has exceeded a detection frequency range and afolded peak frequency has occurred.

BACKGROUND ART

An FM-CW radar measures the distance to a target, such as a vehicletraveling in front, by transmitting a continuous wavefrequency-modulated in, for example, a triangular pattern. Morespecifically, the transmitted wave from the radar is reflected by thevehicle in front, and the reflected signal is received and mixed with aportion of the transmitted signal to produce a beat signal (radarsignal). This beat signal is fast Fourier transformed to analyze thefrequency. The frequency-analyzed beat signal exhibits a peak, at whichthe power becomes large, in correspondence with the target. Thefrequency corresponding to this peak is called the peak frequency. Thepeak frequency carries information about distance, and the peakfrequency differs between the upsweep and downsweep sections of thetriangular FM-CW wave because of the Doppler effect associated with therelative velocity with respect to the vehicle traveling in front. Thedistance and the relative velocity with respect to the vehicle travelingin front can be obtained from the peak frequencies in the upsweep anddownsweep sections. If there is more than one vehicle traveling infront, a pair of peak frequencies in the upsweep and downsweep sectionsis generated for each vehicle. Forming such peak frequency pairs betweenthe upsweep and downsweep sections is called pairing.

FIGS. 1A to 1C are diagrams for explaining the principle of an FM-CWradar when the relative velocity with respect to the target is 0. Thetransmitted wave is a triangular wave whose frequency changes as shownby a solid line in FIG. 1A. In the figure, f₀ is the transmit centerfrequency of the transmitted wave, Δf is the FM modulation amplitude,and Tm is the repetition period. The transmitted wave is reflected fromthe target and received by an antenna; the received wave is shown by adashed line in FIG. 1A. The round trip time T to and from the target isgiven by T=2r/C, where r is the distance (range) to the target and C isthe velocity of propagation of the radio wave.

Here, the received wave is shifted in frequency from the transmittedsignal (i.e., produces a beat) according to the distance between theradar and the target.

The frequency component fb of the beat signal can be expressed by thefollowing equation.fb=fr=(4·Δf/C·Tm)r  (1)where fr is the frequency due to the range (distance).

FIGS. 2A to 2C, on the other hand, are diagrams for explaining theprinciple of an FM-CW radar when the relative velocity with respect tothe target is v. The frequency of the transmitted wave changes as shownby a solid line in FIG. 2A. The transmitted wave is reflected from thetarget and received by an antenna; the received wave is shown by adashed line in FIG. 2A. Here, the received wave is shifted in frequencyfrom the transmitted signal (i.e., produces a beat) according to thedistance between the radar and the target. In this case, as the relativevelocity with respect to the target is v, a Doppler shift occurs, andthe beat frequency component fb can be expressed by the followingequation.fb=fr±fd=(4·Δf/C·Tm)r±(2·f ₀ /C)v  (2)where fr is the frequency due to the range, and fd is the frequency dueto the velocity.

In the above equation, the peak frequency fbup in the upsweep sectionand the peak frequency fbdn in the downsweep section are given byfbup=fr−fd=(4·Δf/C·Tm)r−(2·f ₀ /C)v  (3)fbdn=fr+fd=(4·Δf/C·Tm)r+(2·f ₀ /C)v  (4)

The symbols in the above equations have the following meanings.

fb: Transmit/receive beat frequency

fr: Range (distance) frequency

fd: Velocity frequency

f₀: Center frequency of transmitted wave

Δf: Frequency modulation amplitude

Tm: Period of modulation wave

C: Velocity of light

T: Round trip time of radio wave to and from target object

r: Range (distance) to target object

v: Relative velocity with respect to target object

FIG. 3 is a diagram showing one configuration example of an FM-CW radar.As shown, a modulating signal generator 1 applies a modulating signal toa voltage-controlled oscillator 2 for frequency modulation, and thefrequency-modulated wave is transmitted out from a transmitting antennaAT, while a portion of the transmitted signal is separated and fed intoa frequency converter 3 such as a mixer. The signal reflected from atarget, such as a vehicle traveling in front, is received by a receivingantenna AR, and the received signal is mixed with the output signal ofthe voltage-controlled oscillator 2 to produce a beat signal. The beatsignal is passed through a baseband filter 4, and is converted by an A/Dconverter 5 into a digital signal; the digital signal is then suppliedto a CPU 6 where signal processing, such as a fast Fourier transform, isapplied to the digital signal to obtain the distance and the relativevelocity.

From the above equations (3) and (4)fr=(fbdn+fbup)/2

Since fr=(4·Δf/C·Tm)r, the relative distance r is given byr=(C·Tm/8·Δf)(fbdn+fbup)  (5)

Similarly, from the above equations (3) and (4)fd=(fbdn−fbup)/2

Since fd=(2·f₀/C)v, the relative velocity v is given byv=(C/4f ₀)(fbdn−fbup)  (6)

As can be seen from the above equations (5) and (6), the relativevelocity v is proportional to the difference between fbdn and fbup, andthe relative distance r is proportional to the sum of fbdn and fbup.Therefore, the values of fbdn and fbup decrease as the relative distancer decreases.

FIGS. 4A to 4C are diagrams showing the positional relationship betweenthe upsweep and downsweep peak frequencies when there is a targetapproaching at a high relative velocity and the relative distance istherefore rapidly decreasing. In the figures, the relative distance israpidly decreasing as shown in FIGS. 4A, 4B, and 4C in this order. Whenthe target is approaching at a high relative velocity, the differencebetween the upsweep and downsweep peak frequencies fbup and fbdnincreases. On the other hand, when the relative distance decreases, thevalues of fbup and fbdn decrease; therefore, the values of fbdn and fbupapproach zero as shown in FIGS. 4A, 4B, and 4C in this order, andeventually, the upsweep peak frequency fbup enters the negativefrequency range as shown in FIG. 4C. If this happens, the upsweep peakfrequency fbup can no longer be detected, resulting in an inability todetect the target. Furthermore, when the upsweep peak frequency fbupenters the negative frequency range, a peak due to a folded frequencyf′bup occurs as shown by a dashed line and, as a result, erroneouspairing is done, thus resulting in erroneous measurements of therelative distance and the relative velocity.

FIGS. 5A to 5C are diagrams showing the positional relationship betweenthe upsweep and downsweep peak frequencies when there is a targetreceding at a high relative velocity and the relative distance istherefore rapidly increasing. In the figures, the relative distance israpidly increasing as shown in FIGS. 5A, 5B, and 5C in this order. Whenthe target is receding at a high relative velocity, the differencebetween the upsweep and downsweep peak frequencies fbup and fbdnincreases. On the other hand, when the relative distance increases, thevalues of fbup and fbdn increase; therefore, the values of fbdn and fbupincrease as shown in FIGS. 5A, 5B, and 5C in this order, and eventually,the upsweep peak frequency fbup exceeds the detection frequency range fxas shown in FIG. 5C. If this happens, the upsweep peak frequency fbupcan no longer be detected, resulting in an inability to detect thetarget. Furthermore, when the upsweep peak frequency fbup exceeds thedetection frequency range, a peak due to a folded frequency f′bup occursas shown by a dashed line, and as a result, erroneous pairing is done,thus resulting in erroneous measurements of the relative distance andthe relative velocity.

In a prior art signal processing apparatus for an FM-CW radar, thefolded peak frequency is detected by analyzing the frequency obtainedwhen the sampling frequency is set to one half of the normal samplingfrequency, and pairing is done between the upsweep and downsweep peakfrequencies after converting the folded peak frequency into a peakfrequency that would be obtained if there were no frequency folding (forexample, refer to Japanese Unexamined Patent Publication No.H11-271426).

Further, a pulse-repetition frequency-pulse Doppler radar is disclosedthat measures a correct distance by avoiding the influence of thefrequency folding (for example, refer to Japanese Examined PatentPublication No. H06-70673).

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a signal processingmethod for an FM-CW radar that can accurately detect the relativedistance, relative velocity, etc. with respect to a target approachingor receding at a high relative velocity.

According to the signal processing method for an FM-CW radar of thepresent invention, predicted values for peak frequencies currentlydetected in upsweep and downsweep sections are computed from thepreviously detected relative distance and relative velocity, and it isdetermined whether any of the predicted values exceeds a detectionfrequency range and, if there is a peak frequency that exceeds thedetection frequency range, the frequency is folded and the foldedfrequency is taken as a predicted value, the method then proceeds tosearch the currently detected peak frequencies to determine whetherthere are upsweep and downsweep peak frequencies approximately equal tothe predicted values and, if such upsweep and downsweep peak frequencyare found, the peak frequency approximately equal to the foldedpredicted value is folded and the folded peak frequency is used.

Further, according to the signal processing method for FM-CW radar ofthe present invention, relative distance (r_(a)) and relative velocity(v_(a)) are obtained based on the peak frequencies occurring in theupsweep and downsweep sections,

relative distance (r_(b)) and relative velocity (v_(b)) are computed byfolding one or the other of the peak frequencies occurring in theupsweep and downsweep sections,

when the values of the relative distance (r_(b)) and the relativevelocity (v_(b)) are outside a prescribed range, instantaneous errors(Δr_(a) and Δr_(b)) for the relative distances (r_(a) and r_(b)) areobtained,

integrated values (ΣΔr_(a) and ΣΔr_(b)) are obtained for the respectiveinstantaneous errors, and

when neither Δr_(b)≧Δr_(a) nor ΣΔr_(b)≧ΣΔr_(a) holds, the relativedistance (r_(b)) and the relative velocity (v_(b)) computed by foldingthe peak frequency are employed.

On the other hand, when both Δr_(b)≧Δr_(a) and ΣΔr_(b)≧ΣΔr_(a) hold, therelative distance (r_(a)) and the relative velocity (v_(a)) obtainedwithout folding any peak frequency are employed.

When one or the other of Δr_(b)≧Δr_(a) and ΣΔr_(b)≧ΣΔr_(a) does nothold, a determination as to which data is to be employed is not madeuntil the next cycle.

Further, according to the signal processing method for an FM-CW radar ofthe present invention, relative distance (r_(a)) and relative velocity(v_(a)) are obtained based on the peak frequencies occurring in theupsweep and downsweep sections,

relative distance (r_(b)) and relative velocity (v_(b)) are computed byfolding one or the other of the peak frequencies occurring in theupsweep and downsweep sections and, when the value of the relativedistance (r_(b)) is within a prescribed range, the relative distance(r_(a)) and the relative velocity (v_(a)) obtained without folding anypeak frequency are employed.

According to the present invention, even when the peak frequency in theupsweep or downsweep section has exceeded the detection frequency rangeand a folded peak frequency has occurred, correct pairing can be done,so that the relative distance and relative velocity with respect to thetarget can be accurately detected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are diagrams for explaining the principle of an FM-CWradar when the relative velocity with respect to a target is 0.

FIGS. 2A to 2C are diagrams for explaining the principle of FM-CW radarwhen the relative velocity with respect to a target is v.

FIG. 3 is a diagram showing one configuration example of an FM-CW radar.

FIGS. 4A to 4C are diagrams showing the positional relationship betweenupsweep and downsweep peak frequencies when there is a targetapproaching at a high relative velocity and the relative distance istherefore rapidly decreasing.

FIGS. 5A to 5C are diagrams showing the positional relationship betweenupsweep and downsweep peak frequencies when there is a target recedingat a high relative velocity and the relative distance is thereforerapidly increasing.

FIG. 6 is a flowchart showing an embodiment according to the presentinvention.

FIG. 7 is a flowchart showing an embodiment according to the presentinvention.

FIG. 8 is a diagram for explaining an embodiment of the presentinvention.

FIG. 9 is a flowchart showing an embodiment according to the presentinvention.

FIG. 10 is a flowchart showing the embodiment according to the presentinvention.

FIG. 11 is a diagram for explaining an embodiment of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

(1) In the case of a previously detected target, the relative velocity vand the relative distance r are predicted in the following manner. Therelative velocity v is predicted by assuming that the current detectionvalue v_(i) is approximately the same as the previous detection valuev_(i-1), that isv_(i)≈v_(i-1)  (7)

On the other hand, the relative distance r is predicted by assuming thatthe current detection value r_(i) is related to the previous detectionvalue r_(i-1) byr _(i) ≈r _(i-1) +v _(i-1) ·t  (8)where t is the elapsed time between the previous detection and thecurrent detection.

(2) Next, the predicted value fbup_(i) of the peak frequency in theupsweep section and the predicted value fbdn_(i) of the peak frequencyin the downsweep section are obtained by using the earlier givenequations (5) and (6).

That isr _(i)=(C·Tm/8·Δf)(fbdn _(i) +fbup _(i))  (9)andv _(i)=(C/4f ₀)(fbdn _(i) −fbup _(i))  (10)Hencefbup ₁=(4·Δf/C·Tm)r _(i)−(2f ₀ /C)v _(i)  (11)fbdn ₁=(4·Δf/C·Tm)r _(i)+(2f ₀ /C)v _(i)  (12)

The predicted values fbup_(i) and fbdn_(i) of the upsweep and downsweeppeak frequencies in the current detection cycle can thus be computed.

(3) As shown in FIGS. 4A to 4C, when there is a target approaching at ahigh relative velocity, and the relative distance is therefore rapidlydecreasing, the values of fbdn and fbup approach zero, and eventually,the upsweep peak frequency fbup enters the negative frequency range asshown in FIG. 4C. In this case, a peak due to a folded frequency f′bupoccurs; as a result, this frequency is detected, and erroneous pairingis done based on this frequency.

In such cases, in the present invention, fbup_(i) is obtained from theabove equation (11) and, if the resulting value is negative, it isdetermined that the detected f′bup is the folded frequency. Then, −f′bupobtained by inverting the sign is taken as the actual upsweep peakfrequency fbup, and the relative distance r and the relative velocity vare computed from the equations (5) and (6) by substituting −f′bup forfbup and the currently detected value for fbdn.

FIGS. 4A to 4C have been shown above by taking as an example the casewhere the upsweep peak frequency fbup enters the negative frequencyrange but, in the case of a target receding at a high relative velocity,the downsweep peak frequency fbdn may enter the negative frequencyrange. In either case, only one or the other of the peak frequencies canenter the negative frequency range.

(4) As shown in FIGS. 5A to 5C, when there is a target receding at ahigh relative velocity, and the relative distance is therefore rapidlyincreasing, the upsweep peak frequency fbup can exceed the detectionfrequency range fx as shown in FIG. 5C. If this happens, a peak due to afolded frequency f′bup occurs as shown by a dashed line; as a result,this frequency is detected, and erroneous pairing is done based on thisfrequency.

In such cases, in the present invention, fbup_(i) is obtained from theabove equation (11) and, if the resulting value exceeds the detectionfrequency range fx, it is determined that the detected f′bup is thefolded frequency. Then, the actual upsweep peak frequency fbup isobtained, and the relative distance r and the relative velocity v arecomputed by using the thus obtained actual upsweep peak frequency fbupand the currently detected downsweep peak frequency fbdn.

The actual upsweep peak frequency fbup is obtained in the followingmanner. In FIG. 5C, when the upper limit frequency of the detectionfrequency range is denoted by fx, the actual upsweep peak frequency fbupis given byfbup=fx+(fx−f′bup)Therefore, the relative distance r and the relative velocity v arecomputed from the equations (5) and (6) by substituting fx+(fx−f′bup)for fbup and the currently detected value for fbdn.

EMBODIMENT 1

[The Case of a Previously Detected Target]

FIG. 6 is a flowchart showing an embodiment according to the presentinvention for the case of a previously detected target. The sequence ofoperations shown in the flowchart is controlled by a CPU contained inthe radar apparatus, for example, the CPU 5 shown in FIG. 3.

In FIG. 6, when the target detection process is started (S1), it isdetermined whether there is any previously detected target (S2). Ifthere is such a target (Yes), the relative velocity v_(i) and therelative distance r_(i) in the current cycle of the routine arepredicted (S3). The predictions are done using the earlier givenequations (7) and (8).

Next, predicted values fbup_(i) and fbdn_(i) for the upsweep anddownsweep peak frequencies detected in the current cycle of the routineare computed from the equations (11) and (12), respectively (S4). Then,it is determined whether one or the other of the thus computed predictedvalues fbup_(i) and fbdn_(i) is negative or not (S5). If, for example,the predicted value fbup_(i) is negative (Yes), it can be suspected thatthe upsweep peak frequency fbup lies in the negative range as shown inFIG. 4C; therefore, the sign of the negative frequency data fbup_(i)computed as the predicted value is inverted (S6).

Next, the currently detected peak frequencies are searched to see ifthere are peak frequencies approximately equal to the peak frequenciescomputed as the predicted values (S7). Here, the above frequency data(−fbup_(i)) obtained by inverting the sign is used as one of thepredicted values. Then, it is determined whether there are upsweep anddownsweep peak frequencies approximately equal to the predicted values−fbup_(i) and fbdn_(i) (S8). If there are such frequencies (Yes), therelative distance and relative velocity with respect to the target arecomputed by using, out of the currently detected peak frequencies, theupsweep peak frequency f′bup approximately equal to the predicted value−fbup_(i) and the downsweep peak frequency fbdn approximately equal tothe predicted value fbdn_(i) (S9). Here, for the peak frequency f′bupfor which the predicted value is determined to be negative, the detectedpeak frequency is used by inverting the sign of the frequency data.

In the above flowchart, if the answer in S2 or S8 is No, the routine isimmediately terminated.

On the other hand, if the answer in S5 in FIG. 6 is No, that is, ifneither the computed predicted value fbup_(i) nor fbdn_(i) is negative,the currently detected peak frequencies are searched to see if there arepeak frequencies equal to the peak frequencies computed as the predictedvalues (S11). Then, it is determined whether there are upsweep anddownsweep peak frequencies fbup and fbdn approximately equal to thepredicted values (S11). If there are such frequencies (Yes), therelative distance and relative velocity with respect to the target arecomputed by using, out of the currently detected peak frequencies, theupsweep peak frequency fbup approximately equal to its predicted valueand the downsweep peak frequency fbdn approximately equal to itspredicted value (S12). If the answer in S11 is No, the routine isterminated without computing the relative distance or the relativevelocity.

EMBODIMENT 2

[The Case of Previously Detected Target]

FIG. 7 is a flowchart showing another embodiment according to thepresent invention for the case of a previously detected target. Thesequence of operations shown in the flowchart is controlled by a CPUcontained in the radar apparatus, for example, the CPU 5 shown in FIG.3.

In FIG. 7, the operations from S1 to S4 are the same as those shown inFIG. 6. In this flowchart, it is determined whether one or the other ofthe computed predicted values fbup_(i) and fbdn_(i) exceeds thedetection frequency range fx or not (S5). If it does (Yes), that is,when a situation such as that shown in FIG. 5C is suspected, a frequencyf′bup_(i) that occurs when the frequency fbup_(i) as the predicted valueis folded with respect to the frequency fx is obtained from thefollowing equation (S6).fbup _(i) =fx+(fx−f′bup _(i))f′bup _(i)=2fx−fbup _(i)

Next, the currently detected peak frequencies are searched to see ifthere are peak frequencies approximately equal to the peak frequenciescomputed as the predicted values (S7). Here, the folded frequency data(f′bup_(i)) is used as one of the predicted values. Then, it isdetermined whether there are upsweep and downsweep peak frequenciesapproximately equal to the predicted values f′bup_(i) and fbdn_(i) (S8).If there are such frequencies (Yes), the relative distance and relativevelocity with respect to the target are computed by using, out of thecurrently detected peak frequencies, the upsweep peak frequency f′bupapproximately equal to its predicted value and the downsweep peakfrequency fbdn approximately equal to its predicted value (S9). Here,for the upsweep peak frequency for which the predicted value isdetermined to have exceeded the detection frequency range fx, thefrequency fbup that occurs when the detected peak frequency f′bup isfolded with respect to the frequency fx is obtained from the followingequation.fbup=fx+(fx−f′bup)

In the above flowchart, if the answer in S2 or S8 is No, the routine isimmediately terminated.

On the other hand, if the answer in S5 in FIG. 6 is No, that is, ifneither the computed predicted value fbup_(i) nor fbdn_(i) exceeds theupper limit frequency fx, the currently detected peak frequencies aresearched to see if there are peak frequencies approximately equal to thepeak frequencies computed as the predicted values (S10). Then, it isdetermined whether there are upsweep and downsweep peak frequenciesapproximately equal to the predicted values fbup_(i) and fbdn_(i) (S11).If there are such frequencies (Yes), the relative distance and relativevelocity with respect to the target are computed by using, out of thecurrently detected peak frequencies, the upsweep peak frequency fbupapproximately equal to its predicted value and the downsweep peakfrequency fbdn approximately equal to its predicted value (S12). If theanswer in S11 is No, the routine is terminated without computing therelative distance or the relative velocity.

EMBODIMENT 3

[The Case of New Target]

Before describing an embodiment of the present invention for the case ofa new target, the range within which the distance and the relativevelocity can be accurately detected by a radar will be described belowwith reference to the graph of FIG. 8. In FIG. 8, the horizontal axisrepresents the relative velocity (v), and the vertical axis the relativedistance (r) to the target. The right-hand side of the horizontal axisindicates the positive relative velocity (+v), i.e., the target isreceding. The left-hand side of the horizontal axis indicates thenegative relative velocity (−v), i.e., the target is approaching.

In the graph of FIG. 8, +v₀₁ is the relative velocity of a recedingtarget beyond which the relative velocity need not be detected by theradar (region C1); this relative velocity can be set, for example, to150 km/h. This represents a situation where, for example, when theradar-equipped vehicle is stationary, the target is moving away at 150km/h; usually, a target receding faster than this relative velocity neednot be detected by the radar. Moreover, if the target is detected inthis region, the detected data is highly likely to be in error.

On the other hand, −v₀₂ is the relative velocity of an approachingtarget beyond which the relative velocity need not be detected by theradar (region C2); this relative velocity can be set, for example, to300 km/h. This represents a situation where, for example, when theradar-equipped vehicle is traveling at 150 km/h, an oncoming vehicletraveling at 150 km/h is detected; usually, a target approaching fasterthan this relative velocity need not be detected by the radar. Moreover,if the target is detected in this region, the detected data is highlylikely to be in error.

In the graph of FIG. 8, a diamond-shaped region is the region where nofolding occurs, and this region is bounded by the following straightlines, as shown in the figure. This diamond-shaped region is also theregion into which data may be folded.

(1) r=av

(2) r=−av+r_(x)

(3) r=−av

(4) r=av+r_(x)

Here, the straight lines forming the diamond-shaped region show therelationship between the relative distance (r) and the relative velocity(v) obtained when the frequency in either the upsweep or the downsweepsection changes while holding the frequency in the other section atzero, and can vary according to each individual radar.

The value r_(x) of an upper vertex of the diamond on the vertical axisindicates the distance limit within which no folding occurs when therelative velocity is zero. Accordingly, the third embodiment deals withthe case where the relative distance is within the distance limit r_(x).

The point at which the diagonal extending in the horizontal direction ofthe diamond intersects the vertical axis is denoted by r₀. Further,between the point at which the straight lines v=v₀₁ and r=av intersectand the point at which the straight lines v=−v₀₂ and r=−av intersect,the point that yields the greater relative distance value r is found,and the relative distance value at this point is denoted by r=r₀₁. Onthe other hand, between the point at which the straight lines v=v₀₁ andr=−av+r_(x) intersect and the point at which the straight lines v=−v₀₂and r=av+r_(x) intersect, the point that yields the smaller relativedistance value r is found, and the relative distance value at this pointis denoted by r=r₀₂.

Then, the region defined by the relations −v₀₂≦v≦v₀₁ and r₀₂>r>r₀₁ isdenoted by A (A1, A2). The region A is the region where no foldingoccurs and where there is no possibility of folded data entering it.

On the other hand, in the region B defined by the relations −v₀₂≦v≦v₀₁and r₀₁≧r≧0, the sub-region B2 outside the diamond is the region intowhich the lower frequency is folded, and the sub-region B1 inside thediamond is the region that may contain folded data.

Further, in the region defined by the relations −v₀₂≦v≦v₀₁ andr_(x)≧r≧r₀₂, the sub-region B4 outside the diamond is the region intowhich the higher frequency is folded, and the sub-region B3 inside thediamond is the region that may contain folded data.

FIGS. 9 and 10 are flowcharts showing the embodiment according to thepresent invention for the case of a new target. The sequence ofoperations shown in the flowcharts is controlled by a CPU contained inthe radar apparatus, for example, the CPU 3 shown in FIG. 3.

In the flowchart of FIG. 9, when the target detection process is started(S1), it is determined whether there is any previously detected target(S2). If there is such a target (Yes), the sequence of operations shownin FIG. 6 or 7 is performed.

If it is determined that there is no previously detected target (No inS2), pairing is done between the peak frequencies detected in theupsweep and downsweep sections (S3). Then, the distance (r_(a)) andrelative velocity (v_(a)) with respect to the target are obtained basedon the pairing (S4).

Next, it is determined whether or not the thus obtained distance r_(a)is less than or equal to the predetermined value r₀ (see FIG. 8) (S5).If r_(a)≦r₀ (Yes), the lower of the peak frequencies is folded (S6). Onthe other hand, if the relation r_(a)≦r₀ does not hold (No), the higherof the peak frequencies is folded (S7). Then, distance (r_(b)) andrelative velocity (v_(b)) are obtained based on the folded peakfrequency (S8).

Next, it is determined whether the thus obtained relative velocity(v_(b)) is within a prescribed range (S9). The prescribed range in thiscase is the range where the relative velocity (v_(b)) is neither in theregion C1 nor in the region C2 in FIG. 8, that is, the range defined bythe relation −v₀₂≦v_(b)≦v₀₁.

If the obtained relative velocity (v_(b)) is not within the prescribedrange (No), the distance (r_(a)) and relative velocity (v_(a)) obtainedwithout folding are employed (S22 in FIG. 10). On the other hand, if therelative velocity (v_(b)) is within the prescribed range (Yes), then itis determined whether or not the obtained distance (r_(b)) is less thanor equal to the predetermined value r₀ (S10).

If the obtained distance (r_(b)) is less than or equal to thepredetermined value r₀, that is, r_(b)≦r₀ (Yes), it is determinedwhether the obtained distance is greater than the predetermined valuer₀₁, that is, r_(b)>r₀₁ or not (S11). If r_(b)>r₀₁ (Yes), the obtaineddistance is contained in the region A1 indicated by oblique hatching inFIG. 8; therefore, the distance (r_(a)) and relative velocity (v_(a))obtained without folding are employed (S22 in FIG. 10).

On the other hand, if the answer in S11 is No, the obtained distance iscontained in the region B1 or B2 shown in FIG. 8; therefore, the processproceeds to S13 in the flowchart of FIG. 10.

If, in S10, the obtained distance (r_(b)) is greater than thepredetermined value r₀, that is, if the relation r_(b)≦r₀ does not hold(No), then it is determined whether the obtained distance is smallerthan the predetermined value r₀₂, that is, r_(b)<r₀₂ or not (S12). Ifr_(b)<r₀₂ (Yes), the obtained distance is contained in the region A2indicated by oblique hatching in FIG. 8; therefore, the distance (r_(a))and relative velocity (v_(a)) obtained without folding are employed (S22in FIG. 10).

On the other hand, if the answer in S12 is No, the obtained distance iscontained in the region B3 or B4 shown in FIG. 8; therefore, the processproceeds to S13 in the flowchart of FIG. 10.

If the answer in S11 or S12 is No, the relative distance r_(b) iscontained in the region B (B1, B2, B3, B4) shown in FIG. 8. In thiscase, it is determined whether the distance (r_(a), r_(b)) and therelative velocity (v_(a), v_(b)) are calculated for the first time(S13).

If the distance and the relative velocity are calculated for the firsttime (Yes in S13), the distance (r_(a)) and relative velocity (v_(a))obtained based on the pairing and the distance (r_(b)) and relativevelocity (v_(b)) obtained by folding the peak frequency are stored(S14).

If the distance and the relative velocity are not ones calculated forthe first time (No in S13), an instantaneous error Δr_(a) between thepreviously obtained distance (r_(ai-1)) and the currently obtaineddistance (r_(ai)) is computed from the following equation (S15).Δr _(a)={(v _(ai) +v _(ai-1))/2)}t−(r _(ai) −r _(ai-1))

In the above equation, v_(ai) is the currently obtained relativevelocity, and v_(ai-1) is the previously obtained relative velocity.

Similarly, an instantaneous error Δr_(b) between the distance (r_(bi-1))previously obtained by folding the peak frequency and the distance(r_(bi)) currently obtained by folding the peak frequency is computedfrom the following equation (S10).Δr _(b)={(v _(bi) +v _(bi-1))/2)}t−(r _(bi) −r _(bi-1))

In the above equation, v_(bi) is the relative velocity currentlyobtained by folding the peak frequency, and v_(bi-1) is the relativevelocity previously obtained by folding the peak frequency.

Next, integrated values ΣΔr_(a) and ΣΔr_(b) of the respectiveinstantaneous distance errors (Δr_(a)) and (Δr_(b)) are computed (S16).

Then, it is determined whether the relation Δr_(b)≧Δr_(a) holds or not(S17); if the relation Δr_(b)≧Δr_(a) does not hold (No), then it isdetermined whether the relation ΣΔr_(b)≧ΣΔr_(a) holds or not (S18). Ifthe relation ΣΔr_(b)≧ΣΔr_(a) does not hold (No), the distance (r_(b))and relative velocity (v_(b)) obtained by folding the peak frequency areemployed (S19), because the instantaneous distance errors (Δr_(b))obtained by folding the peak frequency and its integrated value(ΣΔr_(b)) are both smaller than the instantaneous distance error(Δr_(a)) obtained without folding and its integrated value (ΣΔr_(a)).

On the other hand, if the answer in S17 is Yes, that is, ifΔr_(b)≧Δr_(a), then it is determined whether the relationΣΔr_(b)≧ΣΔr_(a) holds or not (S20) and, if the answer is No, which datais to be employed is determined in the next cycle (S21). Likewise, ifΣΔr_(b)≧ΣΔr_(a) holds in S18 (Yes), which data is to be employed is alsodetermined in the next cycle (S21).

If ΣΔr_(b)≧ΣΔr_(a) holds in S20 (Yes), the distance (r_(a)) and relativevelocity (v_(a)) obtained without folding are employed as the data(S22), because the instantaneous distance errors (Δr_(a)) obtainedwithout folding and its integrated value (ΣΔr_(a)) are both smaller thanthe instantaneous distance error (Δr_(b)) obtained by folding the peakfrequency and its integrated value (ΣΔr_(b)).

Here, as indicated in S21, if one or the other of the relationsΔr_(b)≧Δr_(a) and ΣΔr_(b)≧ΣΔr_(a) does not hold, the determination as towhich data is to be employed is not made here, but is made in the nextcycle.

In the third embodiment, the regions C1 and C2 have been defined asregions outside the thresholds of +150 km/h and −300 km/h, respectively,irrespective of the relative distance, but these threshold values may bevaried according to the relative distance. For example, the lower limitof region C1 may be set to +150 km/h when the relative distance is 0,but reduced to +100 km/h when the relative distance is r_(x). In thiscase, the amount of processing can be reduced by not detecting ahigh-speed receding target at long range. Further, C1 and C2 may each bedefined by a boundary line or a curve or the like whose value variesstepwise according to the relative distance.

Further, the third embodiment has been described by dealing with therange where the relative distance is not greater than r_(x) in FIG. 8,in order to prevent the folding determination process from becoming toocomplex, but the present invention is also applicable to the case wherethe relative distance is greater than r_(x) as shown in FIG. 11. In thatcase, a second diamond-shaped region similar to the one shown in FIG. 8is formed above the first diamond-shaped region, and regions A, B, and Care formed in the same manner as in FIG. 8.

In FIG. 11, A3 and A4 correspond to A1 and A2, respectively, B5 and B6correspond to B1 and B2, respectively, and B7 and B8 correspond to B3and B4, respectively.

Here, the region B6 is the region into which the lower or higherfrequency is folded, and the region B5 is the region into which thelower or higher frequency or both frequencies are folded.

In the region B6, from among the data obtained without folding, the dataobtained by folding the lower frequency, and the data obtained byfolding the higher frequency, the data that has the smallest errorshould be employed; on the other hand, in the region B5, from among theabove three data plus the data obtained by folding both frequencies, thedata that has the smallest error should be employed.

When the upper limit of the relative distance is set to r_(x), there isno need to consider frequency folding in the regions A1 and A2, but whenthe upper limit is set to 2r_(x), frequency folding from the regionabove r_(x) can enter the region below r_(x); therefore, in the regionsA1, A2, and B1 to B4, there arises a need to determine whether there isany folding from the region above r_(x).

1. A signal processing method for an FM-CW radar which determines arelative distance and a relative velocity with respect to a target frompeak frequencies occurring in an upsweep section and a downsweep sectionof a triangular FM-CW wave wherein, when said target is a target thathas previously been detected, predicted values for the peak frequenciescurrently detected in the upsweep and downsweep sections are computedfrom the relative distance and relative velocity previously detectedwith respect to said target, and it is determined whether any of saidpredicted values exceeds a detection frequency range, and if there is apeak frequency that exceeds said detection frequency range, saidfrequency is folded and said folded frequency is taken as one of saidpredicted values, said method then proceeding to search the currentlydetected peak frequencies to determine whether there are upsweep anddownsweep peak frequencies substantially equal to said predicted values,and if said substantially equal frequencies are found, said foundfrequencies are folded and said folded frequencies are used to determinethe relative distance and the relative velocity.
 2. A signal processingmethod for an FM-CW radar as claimed in claim 1 wherein, in the case ofa peak frequency for which a predicted value is negative, said predictedvalue is inverted in sign and taken as one of said predicted values,said method then proceeding to search the currently detected peakfrequencies to determine whether there are upsweep and downsweep peakfrequencies approximately equal to said predicted values, and if saidupsweep and downsweep peak frequencies are found, said found frequenciesare inverted in sign and then used to determine the relative distanceand the relative velocity.
 3. A signal processing method for an FM-CWradar as claimed in claim 1 wherein, in the case of a peak frequency forwhich a predicted value exceeds an upper limit frequency of saiddetection frequency range, said peak frequency is folded with respect tosaid upper limit frequency and said folded peak frequency is taken asone of said predicted values, said method then proceeding to search thecurrently detected peak frequencies to determine whether there areupsweep and downsweep peak frequencies substantially equal to saidpredicted values, and if said upsweep and downsweep peak frequencies arefound, said found frequencies are folded with respect to said upperlimit frequency and said folded peak frequencies are used to determinethe relative distance and the relative velocity.